This invention relates generally to current regulators and, more particularly, to current regulators for regulating current in linear or rotating alternating current machines.
Various types of electric motors, such as direct current (DC) and alternating current (AC) motors, are in existence. Although each type of motor has some unique characteristic that renders it suitable for a particular task, each type also has its drawbacks and no one type of motor is suitable for all applications.
DC motors are very useful and versatile because, with such motors, it is very easy to independently control motor speed and torque. However, DC motors require the use of a commutator and its associated brushes. These mechanical elements increase motor cost and complexity, reduce overall ruggedness and reliability and require frequent maintenance.
AC motors, on the other hand, are simpler and more economical in construction, and are more rugged and reliable in operation, than DC motors. However, AC motors are more sluggish in operation than their DC counterparts. Furthermore, it is difficult to control motor speed and torque independently in AC motors. These drawbacks sometime outweigh the advantages of AC motors in certain applications.
In an effort to obtain the operational advantages of DC motors together with the mechanical and economic advantages of AC motors, various controllers for AC motors have been developed. Typically, these controllers have current regulators that vary the instantaneous current to the AC motor in response to a command so as to achieve a desired characteristic in motor operation. In one type of current regulator, a plurality of electronic power switches are coupled to a source of electrical current. A control circuit selectively enables various ones of the electronic switches to change the instantaneous phase and magnitude of the current applied to the AC motor. By properly controlling the phase and magnitude of the motor current, it is possible to control such operational characteristics as torque and motor speed somewhat independently. In this manner, it is possible to mimic DC motor operation using an AC motor. However, because of the large input inductance presented by an AC motor, any abrupt or large magnitude changes in motor current can result in large induced countervoltages or "back EMFs." Unless carefully limited, such back EMFs can destroy the electronic switching devices.
To minimize the magnitude of undesirable back EMFs, some current regulators employ "soft switching" wherein the states of the switching devices are only changed when the instantaneous motor current is near zero. Although this avoids the production of large induced back EMFs, soft switching greatly reduces the maximum effective switching frequency and drastically limits the range of motor response characteristics that can be achieved with "soft switching" systems. "Hard switching" on the other hand, which permits the switching devices to change state even when motor current is non-zero, provides a greater range of achievable motor response characteristics. However, because hard switching can result in abrupt current changes and consequently large induced back EMFs, hard switching is difficult to implement in actual practice.
Typically, current regulators are "load specific" in that they are tailored to (and only work with) motors having specific electrical input specifications. Such regulators are difficult and time consuming to adapt for use with any motor other than the one for which they are designed.
A need exists, therefore, for a switching current regulator that can provide the versatility and responsiveness of a "hard switching" system with the reliability and durability of a "soft switching" system. Such a current regulator working in conjunction with a controller will better enable an AC motor to match the operational characteristics of a DC motor and thereby enhance the versatility of otherwise rugged and reliable AC motors. By making a current regulator "load independent" (i.e., operational with a variety of motors having varying electrical input specifications) system versatility can be further enhanced.
In view of the foregoing, it is a general object of the present invention to provide a new and improved current regulator for induction devices such as AC motors.
It is a further object of the present invention to provide a new and improved current regulator that provides a wide range of attainable, motor operational response characteristics
It is a further object of the present invention to provide a new and improved current regulator that is substantially load independent.
It is a further object of the present invention to provide a new and improved current regulator that permits enhanced and substantially independent control of such AC motor operational characteristics as torque and speed.
It is a further object of the present invention to provide a new and improved current regulator that permits hard switching to further increase system versatility and effectiveness.